doc: add fn main

Closes #227

README.md

@@ -17,7 +17,11 @@ When MoonBit reaches beta, it means any backwards-incompatible changes will be s
 
 ## Overview
 
-A MoonBit program consists of type definitions, function definitions, and variable bindings. The entry point of every package is a special `init` function. The `init` function is special in two aspects:
+A MoonBit program consists of type definitions, function definitions, and variable bindings. 
+
+### Program entrance
+
+There is a specialized function called `init` function. The `init` function is special in two aspects:
 
 1. There can be multiple `init` functions in the same package.
 2. An `init` function can't be explicitly called or referred to by other functions. Instead, all `init` functions will be implicitly called when initializing a package. Therefore, `init` functions should only consist of statements.
@@ -34,6 +38,16 @@ fn init {
 }
 ```
 
+For WebAssembly backend, it means that it will be executed **before** the instance is available, meaning that the FFIs that relies on the instance's exportations can not be used at this stage;
+for JavaScript backend, it means that it will be executed during the importation stage.
+
+There is another specialized function called `main` function. The `main` function is the main entrance of the program, and it will be executed after the initialization stage.
+Only packages that are `main` packages can define such `main` function. Check out [build system tutorial](./build-system-tutorial.md) for detail.
+
+The two functions above need to drop the parameter list and the return type.
+
+### Expressions and Statements
+
 MoonBit distinguishes between statements and expressions. In a function body, only the last clause should be an expression, which serves as a return value. For example:
 
 ```moonbit live
@@ -54,8 +68,6 @@ fn init {
 }
 ```
 
-### Expressions and Statements
-
 Expressions include:
 
 - Value literals (e.g. Boolean values, numbers, characters, strings, arrays, tuples, structs)

zh-docs/README.md

@@ -17,7 +17,10 @@ MoonBit 目前处于 Pre-alpha 阶段，是实验性质的。我们期望今年
 ## 概述
 
 一个月兔程序由类型定义，函数定义和变量绑定组成。
-每个包的入口点是一个特殊的 `init` 函数，它有以下两个特点：
+
+### 程序入口
+
+有一个特殊的函数： `init` 函数，它有以下两个特点：
 
 1. 同一个包中可以有多个 `init` 函数。
 2. `init` 函数不能被显式调用或被其他函数引用。相反，在一个包初始化时，所有的 `init` 函数都将被隐式地调用。因此，`init` 函数中只能包含语句。
@@ -34,6 +37,14 @@ fn init {
 }
 ```
 
+对于WebAssembly后端而言，这意味着它将会在实例准备好**之前**被执行，也就是说，如果有FFI依赖实例的导出，那么将不能正常运行；对于JavaScript后端而言，这意味着它将会在被导入的时候执行。
+
+另一个特殊的函数是`main`函数。`main`函数是程序的入口，它将会在初始化阶段之后被执行。只有是`main`的包中才能定义`main`函数。查看[构建系统教程](./build-system-tutorial.md)了解更多。
+
+上述两个函数均需省略参数列表与返回值定义。
+
+### 表达式和语句
+
 MoonBit 区分语句和表达式。在一个函数体中，只有最后一句才能写成作为返回值的表达式。例如：
 
 ```moonbit live
@@ -54,8 +65,6 @@ fn init {
 }
 ```
 
-### 表达式和语句
-
 表达式包括：
 
 - 值字面量（例如布尔值、数字、字符、字符串、数组、元组、结构体）